1. Field of the Invention
This invention relates to solid electrolytic capacitors using a conductive polymer layer as a solid electrolyte and methods for manufacturing the same.
2. Description of Related Arts
With the recent trend towards smaller and lighter electronic devices, high-frequency capacitors of small size and high capacity are being desired. Among the capacitors proposed as such high-frequency capacitors are solid electrolytic capacitors in which the surface of an anode formed of a sintered body of a valve metal, such as tantalum, niobium, titanium or aluminum, is anodized to form a dielectric layer and a solid electrolyte layer is provided on the dielectric layer. In these solid electrolytic capacitors, a conductive polymer is used as the solid electrolyte layer to seek to reduce the equivalent series resistance (ESR).
However, the above solid electrolytic capacitors have a problem in that if the adherence between the dielectric layer formed of an inorganic material and the conductive polymer layer formed of an organic material decreases, the ESR increases.
To solve the above problem, Published Japanese Patent Applications Nos. H02-74021, H04-73924 and H08-293436 propose techniques in which after the formation of a dielectric layer on the surface of an anode, the surface of the dielectric layer is treated with a silane coupling agent and a conductive polymer layer is then formed thereon.
Published Japanese Patent Application No. H11-329900 proposes a technique of repeatedly performing a treatment with a silane coupling agent to the surface of a dielectric layer and a subsequent formation of a conductive polymer layer thereon.
Published Japanese Patent Application No. 2006-140443 proposes a technique in which a first conductive polymer layer is formed partially on a dielectric layer, a silane coupling agent-treated layer is then formed on part of the dielectric layer on which the first conductive polymer is not formed, and a second conductive polymer layer is formed on both the first conductive polymer layer and the silane coupling agent-treated layer.
The silane coupling agents used in the techniques disclosed in Published Japanese Patent Applications Nos. H02-74021, H04-73924, H08-293436, H11-329900 and 2006-140443 have a molecular structure containing a coupling group capable of being bonded to the dielectric layer made of an inorganic material and a hydrophobic group of good wettability with the conductive polymer made of an organic material. According to these techniques, the adherence between the silane coupling agent serving as a surface treatment and the conductive polymer serving as a solid electrolyte is improved but is not satisfactory.
A solution to the above is proposed in Published Japanese Patent Application No. 2006-140442, wherein the improvement in adherence is implemented by using a surface treatment having a molecular structure in which a group serving as a dopant for conductive polymer is located at a distal end of a hydrophobic group. However, this solution has a problem in that the process of reacting the surface treatment with the dielectric layer is complicated.
Another solution is proposed in Published Japanese Patent Application No. 2009-32895, wherein the improvement in adherence is implemented by using a surface treatment having a molecular structure in which a structure of a conductive polymer monomer is bonded to a distal end of a hydrophobic group.
However, even with the use of the above surface treatments, the reduction in ESR is not yet satisfactory.